This invention relates to the manufacture of fused material having a very small ultimate crystal size by virtue of a novel method of high speed cooling from the molten state. It is intended to apply principally to the manufacture of oxidic abrasive materials such as those commonly produced by the fusion of chemically purified alumina, or by the fusion and partial reduction of bauxite which has been known for many years by the term "regular aluminum oxide", as well as to alumina-zirconia fused abrasive materials containing amounts of zirconia up to and including the eutectic composition, being about 42 percent zirconia by weight. The said alumina-zirconia materials may contain varying amounts of other constituents commonly associated with bauxite, such as iron oxide, titanium oxide, and silicon oxide. The novel process disclosed herein is, however, not limited to the compositions set forth above.
The fusion and reduction of bauxite for abrasive purposes has long been known, following the teaching of U.S. Pat. No. 659,926 to Jacobs (Oct. 16, 1900). The manufacture of alumina-zirconia abrasive materials is likewise old, as shown by U.S. Pat. Nos. 1,240,490 and 1,240,491 to Saunders et al (both Sept. 18, 1917) as well as by U.S. Pat. No. 3,181,939 to Marshall et al. (May 4, 1965).
It has also been known since the time of U.S. Pat. No. 1,192,709 to Tone (July 25, 1916) that pouring molten aluminous abrasive material from the furnace into a mold so as to freeze the abrasive material relatively quickly will yield a solid abrasive material of small crystal size by virtue of which a durable and strong product will result, having utility in the production of abrasive grains especially suited for heavy grinding. For this reason, much attention has been paid to methods of achieving a fine crystal structure.
More recently, U.S. Pat. No. 3,781,172 to Pett et al. (Dec. 25, 1973) describes pouring a molten abrasive composition over relatively cold lumps of similar aorasive material to achieve fast cooling and fine crystal size. U.S. Pat. No. 3,726,621 to Cichy (Apr. 10, 1973) teaches casting the molten abrasive composition into a plurality of steel balls to obtain a fast cooling rate and fine crystal size. Canadian Pat. No. 956,122 to Scott (Oct. 15, 1974) describes pouring the molten abrasive material into a vessel having a plurality of parallel spaced metal plates therein to give a fine crystal size. Canadian Pat. No. 924,112 to Shurie (Apr. 10, 1973) teaches casting the molten material into a plurality of objects having a relatively high thermal conductivity. U.S. Pat. No. 3,646,713 to Marshall et al. (March 7, 1972) covers an apparatus involving a cool rotating casting cylinder and pressure roll to cool and densify the abrasive material.
All of the methods referred to in the proceding paragraph entail pouring the melted abrasive material from the furnace into vessels or contrivances for subsequent cooling. This requires expensive and complicated furnaces capable of pouring the molten material therefrom, usually by tilting the furnace. The prior methods also involve the fabrication and maintenance of expensive molds or contrivances to handle the poured liquid. The action of molten abrasive materials at temperatures usually in excess of 1800.degree. C. can cause damage to equipment with consequent costly maintenance and frequent replacement. The pouring operations also may be dangerous to the operating personnel.
All of these methods also involve a time loss resulting from the interruption of the charging and fusing operation by the pouring operation. Likewise there is a time lapse between egress of the molten material from the furnace and ingress into the receptable. During this time, heat is lost from the melt. Consequently, it is necessary in any such pouring process, to heat the melt well above its fusion temperature to compensate for unavoidable heat losses in pouring and to prevent premature solidification. This higher temperature requires a corresponding expenditure of extra power in the furnace during melting. Thus, there is a waste of energy.
Another shortcoming in these conventional methods is their relatively high demand of operating labor. In such processes there will be, in addition to one or more furnace operators, extra personnel to place, assemble, empty, clean, and service the receptable equipment. A high production rate requires a correspondingly higher requirement of such extra personnel in addition to a greater investment in the receptable equipment itself.